The present invention relates to a pneumatic tire which is able to obtain a large coefficient of friction on ice, to a method of manufacturing the pneumatic tire which is able to obtain a large coefficient of friction on ice, and to a rubber composition and a vulcanized rubber composition which are able to obtain a large coefficient of friction on ice.
A studless tire has been proposed in which a foam rubber is used for a tread and in which running ability on ice is improved by removing water existing between an icy surface and the ground contact surface of the tread.
Further, in the field of this type of tire, a method of forming closed cells such as a foam rubber and a method of forming microscopic grooves on the surface of a rubber are known as a method for improving coefficient of friction on ice.
As a method of forming microscopic grooves on the surface of a rubber, JP-A No. 4-38207 discloses a foam rubber having short fibers. However, the disclosed fibers are likely to be curled by heat contraction during vulcanization, and as the fibers are pressed into the groove portions, i.e., siping portions, of a mold, the fibers tend to bend in the tread rubber.
As a result, even if the tread wears due to the running of a vehicle, fibers which are not substantially parallel to the wearing surface of the tread do not easily fall out from the rubber. The microscopic grooves, which were intended in the first place, cannot be efficiently formed on the surface of the rubber so that the coefficient of friction on ice is not sufficiently improved. Moreover, the microscopic grooves may be broken in a case in which the load applied to the tire is great.
Moreover, JP-A No. 4-110212 discloses the structure of a pneumatic tire in which tubular fibers are dispersed on a tread rubber and in which water existing between the icy surface and the ground contact surface of the tread is drained by tubular portions of the fibers. However, when the fibers are kneaded with the rubber, the fibers may be crushed due to the pressure, the flowing of the rubber, the temperature, or the like at the time of mixing the rubber. Thus, in actuality, the tubular shape of fibers cannot be maintained, and insufficient drainability may occur.
The present invention was developed in light of the drawbacks generated from the above-described conventional arts, and the object thereof is to provide a pneumatic tire, a method of manufacturing the pneumatic tire, a rubber composition, and a vulcanized rubber composition having excellent drainability of the water membrane formed between an icy surface and a tread of the tire and an increased coefficient of friction on the icy surface.
A first aspect of the present invention is a pneumatic tire in which a belt layer and a tread rubber are disposed at an outer circumference of a crown portion of a carcass layer which extends over a pair of bead cores and is formed in a toroidal shape, wherein: the tread rubber includes an elongated closed cell coated by a protective layer formed by a resin. As a result, when the tread rubber wears due to the running of a vehicle, convex portions formed by elongated closed cells are formed on the ground contact surface. The concave portions serve as draining passages and drain the water on the ground contact surface to provide an increased coefficient of friction on an icy surface. Further, since crushing of the concave portions is prevented by resinous protective layers, drainability is maintained even at the time of a high load.
In the second aspect of the present invention, a raw tread rubber includes a blowing agent, which generates gas during vulcanization of a tire, and an elongated resin, which is melted or softened during vulcanization of the tire so that the viscosity of the elongated resin is lower than the viscosity of a rubber matrix. The raw tread rubber is adhered to a crown portion of a raw tire casing. Thereafter, the raw tire casing, to which said raw tread rubber is adhered, is formed by vulcanization in a predetermined mold by means of heating and pressurizing.
During vulcanization, until the time when the tread rubber reaches the maximum vulcanizing temperature, the viscosity of the rubber matrix increases due to cross-linking. The rubber matrix described herein indicates a rubber portion other than the elongated resin. The melted elongated resin forms a phase whose viscosity is greatly lowered in the rubber matrix. On the other hand, a blowing agent begins to react, and gas is diffused within the rubber matrix and distributed between the phase of the elongated resin and the phase of the rubber matrix.
At this time, the gas is concentrated on the phase of an elongated resin whose viscosity is relatively lower than that of the rubber matrix. Accordingly, the phase of the elongated resin becomes hollow, and the elongated closed cell whose outer circumferential portion has a protective layer formed by the resin is formed within the tread rubber.
The third aspect of the present invention includes at least an elongated resin and a blowing agent which generates gas during vulcanization of a tire. The viscosity of the elongated resin is lower than the viscosity of the rubber matrix until the time when the rubber composition reaches the maximum vulcanizing temperature of a tread rubber. As a result, when the rubber composition is vulcanized, until the time when the rubber composition reaches the maximum vulcanizing temperature, the viscosity of the elongated resin is lower than that of the rubber matrix. At the same time, the gas is generated by the blowing agent. As the portion of the gas is concentrated on the interior of the elongated resin whose viscosity is lowered, the elongated closed cell whose outer circumferential portion has a protective layer formed by the resin is formed.
In this case, the rubber composition includes the elongated resin whose viscosity is lower than that of the rubber matrix until the time when the rubber composition reaches the maximum vulcanizing temperature of the tread portion and the blowing agent which generates gas during vulcanization. The vulcanized rubber composition is obtained by vulcanizing the rubber composition. The vulcanized rubber composition includes an elongated closed cell whose outer portion is reinforced by a protective layer formed by the resin. If 10% or more of the overall foaming volume of the rubber is occupied by foaming of the elongated closed cells, when the surface of a tread wears, concave portions formed by the elongated closed cells are formed on the worn surface and serve as draining passages. Because water on the ground contact surface is drained by the concave portions, the coefficient of friction on the icy surface improves, and it is difficult for the tire to slip against the icy surface. Moreover, since crushing of the concave portions is prevented by the resinous protective layers, drainability is maintained even at the time of a high load.